classes/image: Use xargs to set file timestamps

Instead of having find directly invoke touch for each file in the root
file system, pass a list to xargs for batching. This significantly
reduces the number of times the touch program is invoked and speeds up
the do_image task time:

    PKG           TASK      ABSDIFF  RELDIFF  CPUTIME1 -> CPUTIME2
    my-image      do_image   -45.3s   -94.2%     48.1s -> 2.8s

    Cumulative cputime:
      -44.3s    -92.3%    00:48.1 (48.1s) -> 00:03.7 (3.7s)

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 15c65f90)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>